1. Technical Field
The invention relates to a method and an apparatus for welding sheets of metal in general, and to methods and apparatus for welding sheets of metal that sense the edges of the sheets of metal along which the sheets are to he welded, in particular.
2. Background Information
It is known to weld so-called tailored blanks, which are then formed into a formed body, from metal sheets which as a rule have dissimilar properties (e.g. thickness, material). Such formed bodies are used e.g. in the motor vehicle industry. Welding of the tailored blanks is performed by, for example, laser or electron-beam welding. In the case of laser-beam welding, in order to obtain a weld seam of sound quality that is fit for the subsequent forming process it is necessary that the sheets when positioned with their edges butted together should have a very small gap between them. This gap should not exceed, for example, 0.08 mm so that welding with a focused laser beam of 0.2 mm diameter can be carried out without any problem. However, when the individual sheet-metal parts are sheared or punched, errors may occur and/or sheets may be distorted because of internal stresses, preventing a gap dimension of 0.08 mm between the sheets from being adhered to. Touching up the edges of all sheets in the course of their fabrication is expensive. Also it is undesirable for space and handling reasons to have to arrange separate machining stations in front of the welding unit. The fundamental problem of the invention, therefore, is to provide a welding method and apparatus for tailored blanks that allow trouble-free welding without the stated drawbacks.
According to the present invention, a method for welding a pair of metal sheets together along an edge of each metal sheet is provided that includes the steps of 1) determining a line for each of the edges to be welded using one or more sensors: 2) selecting one of the edges to be welded as a dominant edge and the other edge to be welded as a non-dominant edge: and 3) welding the metal sheets together along the dominant edge and the non-dominant edge. The present invention further includes an apparatus capable of performing the present method.
Detecting the edge line of both sheets and selecting one of the edges as a datum edge means that only one of the edges need be machined, and the control device can select the dominant or datum edge in such a way that the machining cost is kept as low as possible. The result is a pairing of the sheets in the welding machine, so that a separate machining station is not needed.
In a preferred embodiment, the sheets are not paired off if the edge lines of the two sheets are so different that the machining cost would be excessive. In this case, one of the sheets is discarded and replaced with another. Depending on its edge line, the discarded sheet may be taken out of the production process altogether, or returned to a sheet stack from which it will later be fed to the welding machine again, along with another sheet.